The polymer flow model is employed to predict the transient behavior of the concentration and temperature profiles within the growing particles during gas-phase ethylene polymerization in the absence and presence of n-hexane as the induced condensing agent. It is demonstrated that by accurate estimation of the model parameters, one can precisely describe the impact of gas phase composition on the reaction rate. The simulation results show that the experimentally observed change in the polymerization rate under the various operating conditions is the direct result of alteration in the average concentration of ethylene in the particle; the average ethylene concentration not only depends on its equilibrium concentration but is also controlled by the significance of the mass transfer resistance through the growing particles. This is while the effect of partial pressure of n-hexane and ethylene on the thermal behavior of the particle is found to be insignificant for the studied experiments.