During olefin polymerization on heterogeneous catalyst, a catalyst particle undergoes fragmentation, and the formed polymer gets deposited on the fragments. These polymer-coated fragments (microparticles) together form a porous polymer particle (macroparticle). The multigrain model (MGM) gives a detailed description by accounting for the monomer diffusion phenomena at both levels. The original approach to solution involved a sequential shell-by-shell determination of monomer concentration profiles, with both radial boundaries of the shells moving with the particle growth. A fixed boundaly system of simultaneous differential equations enables easier computer implementation of the MGM model. Further, in a new development presented here, the interstitial spaces between the microparticles make up the pores through which monomer transport occurs not only by diffusion, but also by convection. The convection is driven by the pressure gradient created by the monomer consumption within the particle. Consistent with recent experimental observations, significantly higher monomer transport rates are thus predicted.