A comprehensive model has been developed to handle the reactions in polymers undergoing polycondensation reactions in the solid state. The polymer crystalline fraction is modeled as containing only repeat units, thus concentrating end groups and condensate in the amorphous fraction. In addition, by using a general framework for the equations, many previously neglected effects are included for example, variable crystallinity and gas phase mass transfer effects. This model is compared to PET and nylon reaction data with good results.