The fluid flow and heat transfer of CO₂ gas in an internal condensation reactor for methanol synthesis was numerically evaluated. The properties of CO₂ were set to constant (i.e., Const case) and temperature-dependent (i.e., Variable case) conditions. In the Variable case, the fluid flow near the inlet of the reactor was not uniform, and circulated flows occurred in the outer tube. Compared to the temperature distribution in Const case, that in Variable case changed. This is due to the enhanced heat transfer caused by the fluid flow. Considering the temperature dependence of fluid properties, it was shown that the flow in the reactor became complex, and changes in flow behavior also affected the behavior of heat transfer.