In order to control flows and temperature distribution within plasma reactors experimentally requires significant research and development costs. the computational fluid dynamics (CFD) can be used as a powerful tool to investigate the flow pattern and heat transfer inside plasma reactors allowing us to construct best designs with minimal cost. In this work temperature profile and gas flow distribution in the thermal plasma reactor used for carbon nanotubes synthesis is studied by COMSOL Multiphysics. Two background gases Helium (He) and Nitrogen (N2) was compared. We conclude that unlike Nitrogen, Helium ensures a good recirculation in the arc chamber and therefore a good replenishment of the atmosphere by the reactants during the process.