A model of solid-state polymerization (SSP) for nylon 6 has been proposed in considering the effect of segmental diffusion on the reversible step-growth polymerization with a monofunctional acid regulator. This model is an adaptation of Chiu et al. for characterizing the Trommsdorff effect in free-radical polymerization and of Kaushik et al. for the SSP model of spherical pellets of nylon 6. Then it is verified by using the experimental data ourselves. The results of the simulation are found to show qualitative trends as found by the model of Kaushik and Gupta. The effects of model parameters and the initial values of the individual feed conditions on the process of SSP for nylon 6 are also studied.