We propose a method of numerically analyzing a polymerization process, extending for a few hours in a batch-type stirred vessel using modeling flow fields by zoning reaction process. We also propose a method of estimating quality of the product polymer, such as average molecular weight distributions and index, while at the same time simulating the reaction process. The methods are applied to thermal bulk polymerization of styrene in a stirred vessel equipped with a double helical ribbon impeller. We verified that the methods proposed here have enough reliability by ascertaining good agreement of the simulated results with experimental values in the same system as that of the simulation.