An improved mathematical model using the Vrentas-Duda theory for diffusion coefficients is developed for solid-state polymerization (SSP). This model is applied to nylon 6, and best-fit values of the parameters are obtained for this system using the Box complex analysis on available experimental data. The sensitivity of results to variations of these parameters is studied. It is found that the ring opening reaction can easily be omitted and the number of parameters thereby decreased. Detailed quantitative results are obtained to study the effects of changing the important operating conditions on SSP, e.g., intermediate remelting of nylon 6 powder, value of the water concentration (or level of vacuum) in the vapor phase, leaching of monomer and water before SSP, size and degree of crystallinity of polymer particles, etc. The diffusional effects on the rate constants are found to depend on a complex interplay of several important factors such as the generation of monomer and water by reaction, removal of water by macro-level diffusion, changes in the intrinsic rate constants due to decrease in the concentration of polymer molecules, etc.