Solid-phase polymerization (SPP) reactors are used to increase the degree of polymerization (DP) during nylon 6,6 production. In previous articles, a reactor model with partial differential equations (PDEs) in time and two spatial dimensions was developed to describe dynamic changes in polymer property profiles (DP, temperature, and moisture content) over the height of the reactor and within the polymer particles. In the current article, a simplified model is developed by deriving appropriate expressions for heat- and mass-transfer coefficients and performing a lumped heat- and mass-transfer analysis. Using this approach, the radial dimension is removed from the PDEs, so that the effort required to solve the model equations is substantially reduced. Predictions of the complex and simplified models are compared through simulation of two different start-up processes. Good agreement between simplified and complex models is obtained, indicating that the simplified model can be used in place of the complex model if the polymer properties profiles within individual particles are not of particular concern to the model user. (C) 2003 Wiley Periodicals, Inc.