Partially-premixed flames (PPF) can contain multiple reaction zones, e.g., one or two with a premixed-like structure and one being a nonpremixed reaction zone. An intrinsic feature of partially premixed flames pertains to the synergistic interactions between these two types of reaction zones that are characterized by heat and mass transfer between them. Since these interactions are strongly dependent on the distribution of the radical and stable species' concentrations, an accurate representation of the flame chemistry involving these species is critical for simulating their behavior. The role of C-1-chemistry in determining the structure of partially premixed methane-air flames is investigated herein by employing two relatively derailed chemical mechanisms. The first involves only C-1-containing species and consists of 52 reactions involving 17 species, while the second mechanism represents both C-1- and C-2-chemistry and consists of 81 reactions that involve 24 species. A planar two-dimensional partially premixed flame established on a rectangular slot burner is simulated. The simulation is based on the numerical solution of the time-dependent conservation equations for mass continuity, momentum, species, and energy. The computations are validated by comparison with the experimentally-obtained chemiluminescent emission from excited-C-2* free radical species, as well as with velocity measurements using particle image velocimetry. A numerical study is then conducted to examine the applicability of C-1 and C-2 mechanisms for predicting the structure of partially premixed flames for different levels of partial premixing and reactant velocity. Results indicate that both the mechanisms reproduce the global structure of PPF over a wide range of reactant velocity and stoichiometry. Since the C-1 mechanism is known to be inadequate for fuel-rich premixed flames, its relatively good performance can be attributed to the interactions between the two reaction zones that characterize the PPF structure. There are, however, important quantitative differences between the predictions of the two mechanisms. The C-2 mechanism is overall superior compared to the C-1 mechanism in that its predictions are in closer agreement with our experimental results. The rich premixed reaction zone height obtained with the C-2 mechanism is more sensitive to variations in the equivalence ratio as compared with predictions that are obtained using the C-1-mechanism. In addition, for high levels of partial premixing, the methane consumption in the inner reaction zone is significantly increased when the C-2-mechanism is employed. compared to when the C-1-mechanism is used. Consequently, the amount of methane that leaks from the rich premixed to nonpremixed reaction zone is significantly lower when the C-2-mechanism is used. The interactions between the inner and outer reaction zones are stronger when the C-2-mechanism is employed. Finally, the maximum temperature predicted by the C-2-mechanism is slightly lower as compared to that obtained using the C-1-chemistry alone. These differences are attributed to the presence of the C-2-chain in the 81-step mechanism, which strongly affects the inner premixed reaction zone.