An accurate mass transfer model is required to calculate the diffusion coefficient in polymers to describe the mass transfer of volatiles in process intensification, especially in the devolatilization units. However, the diffusion coefficient actually is variate rather than a fixed value due to the changing concentration and viscosity of polymer in the process of devolatilization. In this work, removal rates of the volatiles in liquid film were obtained by the intelligent gravity analysis, and a discrete mathematical model was established to compute the instantaneous dynamic diffusion coefficients. On the basis of a large number of experimental data, a correlation of diffusion coefficients was developed by dimensionless regression analysis with temperature, concentration, viscosity, molar mass, molecular polarity, etc. It can well describe the diffusion process in polymers. Furthermore, a molecular simulation was conducted to explore the proper means of the bonding method and force field in the calculation of the diffusion coefficient.