A powerful new modeling technique is presented for exploring mixing effects in reactive flow systems. The technique combines the detailed mixing representation of computational fluid dynamics (CFD) simulations with the simplicity and computational efficiency of stirred-tank compartment models. The method can be generally applied to many reactive systems, and requires only that appropriate criteria be specified to select compartment models from a single converged CFD simulation. A 1000-fold increase in computational speed is achieved by the new compartment model/CFD technique when modeling steady-state operation of low-density polyethylene (LDPE) autoclave reactors, making broad studies of operating conditions and stability possible. Results from compartment models are shown to capture the accuracy and detail of CFD simulations over a wide range of LDPE reactor operation, and the new technique provides a combination of modeling detail and computational ease not previously available for this type of reactor. (c) 2005 American Institute of Chemical Engineers.