This work presents a dynamic model for a polyesterfication reaction in a system containing a batch-wise operating reaction vessel connected to a flash separation unit. This model includes kinetics, description of the change of rate order during the reaction, the polymer nonrandom theory of liquid (NRTL) nonideal thermodynamic model based on NRTL and mass balances. The reaction between maleic anhydride and propylene glycol has been taken as a case study. The reaction scheme is complex, and the proposed model takes four types of reactions into account, ring opening, polyesterfication, isomerization, and saturation reactions. The acid value of the polyester, number-average molecular weight, distilled mass, and glycol concentration in the distillate have been subsequently used to validate the model, and the model predicts these important variables reliably. The process description is improved by using the vapor-liquid equilibrium data predicted from the polymer NRTL model. Particularly, the prediction of distilled mass [regression coefficient (R(2)) = 0.995] and the prediction of propylene glycol concentration (R(2) = 0.97) in the distillate are significantly improved. POLYM. ENG. SCI., 51:2495-2504, 2011. (C) 2011 Society of Plastics Engineers