Gaseous Fick diffusion coefficients are calculated for the CH4-CO2 system using molecular dynamics. CH4 was modeled using the united-atom transferable potentials for phase equilibria (TraPPE) force field developed by Martin and Siepmann [J. Phys. Chem B. 1998, 102, 2569], and CO2 was modeled using the rigid TraPPE force field developed by Potoff and Siepmann [AIChE J. 2001 47, 1676]. They can adequately predict the compressibility factor of the CH4-CO2 system over a substantial temperature, pressure, and composition range. However, deviations from experimental values are observed at conditions for which the ideal gas mixture assumption is not valid. It is shown that reasonable calculations of the Fick diffusion coefficient are obtained when compared to experimental data, and a correlation is found in the literature at conditions for which the ideal gas mixture behavior is observed. Calculations are also performed where the mixture does not behave as an ideal gas and are compared to calculations from the aforementioned correlation. The methods yield different results, however the lack of experimental data at these conditions does not allow one to say which are more accurate.