The multizone circulating reactor allows the production of intimate mixtures of polyolefins of widely different characteristics, enlarging the envelope for properties of these polymers. A mathematical model for the gas-phase polymerization of olefin in a multizone circulating reactor (MZCR) is presented. The system is modeled as a series of two interconnected polymerization zones working with different gas-phase compositions. The model takes into account particle and reactor levels, as well as the particle population balance. Simulations show that a wide range of product characteristics can be achieved by varying the operation conditions in the reactor and that very active catalysts are required to fully exploit the advantages of this reactor.