The computational fluid dynamics (CFD) approach was adopted to simulate benzoyl peroxide (BPO)-initiated styrene polymerization in a laboratory-scale continuous stirred-tank reactor (CSTR). The CFD results revealed the effects of non-homogeneity and the short-circuiting of the unreacted styrene and initiator on the reactor performance. The study also investigated the effects of the impeller speed and the residence time on the conversion and the flow behavior of the system. The CFD simulation showed that intense mixing remained confined to a small region near the impeller. With increasing impeller speed, it was found that the perfectly mixed region near the impeller expanded, thus reducing non-homogeneity. Different contours were generated and exhibited the effect of the mixing parameters on the propagation rate and styrene conversion. The monomer and initiator conversions predicted with the CFD model were compared to those obtained with a CSTR model. The CFD model accounts for the non-ideality behavior of the polymerization reactor, and hence conversion predictions are more realistic.