The effects of nonuniformity of temperature, physical properties and polymer conversion on the polymerization process in a laminar flow reactor are studied. In the experiments, conversion and average molecular weight of styrene are measured at the outlet of a tubular reactor with heated walls. To theoretically investigate the measured results, the governing equations for momentum, heat and mass transfer with polymerization reaction terms are solved numerically, considering the dependence of the physical properties of the mixture of monomer and polymer on temperature and conversion as well as consumption of monomer and heat generation by the reaction. The calculations show that significant change in the mixture viscosity near the reactor walls due to polymerization causes change in the flow velocity distribution in the reactor. The calculated conversions and average molecular weights agree well with. the measured results when the flow velocity of the mixture in the reactor is relatively large. When the flow is slow, however, different conversions are observed between the mixtures flowing upward and downward in the vertical reactor. Temporal change in measured conversion suggests that the difference is caused by gravitational sedimentation of polymer in the reactor.