Mathematical models for simultaneous reaction and mass transfer occurring in the manufacture of high-viscosity, condensation polymers are considered. The specific example of polycondensation of polyethylene terephthalate is examined. Reactor performance is estimated by using a kinetic expression modified by an effectiveness factor. The effectiveness factor is correlated against a ratio of two characteristic times, one identified as the time scale of mixing and the other identified as the time scale of reaction, The time scale of mixing is estimated from experimental mixing data, thereby avoiding the use of potentially inaccurate mixing assumptions. In place of reaction experiments, overall. reaction rates are generated using a more detailed mixing-cell model. The effectiveness factor correlation is compared against previous models. Because the correlation is based on predicted reaction rates rather than experimentally measured reaction rates, the value of the present work lies in the demonstration of the time scales modeling technique.