A practical methodology for the computer modeling of multicomponent chain-growth polymerizations, namely, free-radical and ionic systems, has been developed. This is an extension of a paper by Hamielec, MacGregor, and Penlidis (Multicomponent; free-radical polymerization in batch, semi-batch and continuous reactors. Makromol. Chem., Macromol, Symp. 1987, 10/11, 521). The approach is general, providing a common model framework which is applicable to many multicomponent systems. Model calculations include conversion of the monomers, multivariable distributions of concentrations of monomers bound in the polymer chains and molecular weights, long- and short-chain branching frequencies, chain microstructure, and crosslinked gel content when applicable. Diffusion-controlled termination, propagation, and initiation reactions are accounted for using the free-volume theory. When necessary, chain-length-dependent diffusion-controlled termination may be employed. Various comonomer systems are used to illustrate the development of practical semibatch and continuous reactor operational policies for the manufacture of copolymers with high quality and productivity. These comprehensive polymerization models may be used by scientists and engineers to reduce the time required to develop new polymer products and advanced production processes for their mannufacture as well as to optimize existing processes.