Abstract: A heat and mass transfer model across the external boundary layer of polymer particles in emulsion phase with condensed mode operation was derived.The overheat possibility of particles was analyzed by using the model.Simulation results indicate that in the early lifetime,there exist multi-steady states of surface temperature of growing particles.Using the condensed mode operation will be helpful to reduce the temperature gradient for larger particles,but not for very small particles.The type of catalysts with a low startup,high activity and quick deactivation was suggested to be used in the condensed mode operations.