Very high temperature reactors (VHTRs) with helium-cooled prismatic cores are one type of Generation IV gas-cooled reactors proposed for implementation in next-generation nuclear power plants. To contribute to the VHTR development, a high-temperature/high-pressure test facility has been constructed and used to investigate the convection heat transfer of gaseous coolants. This test facility consisted of a single flow channel with a diameter of 16.8 mm in a graphite column with a length of 2.7 m (9 ft) equipped with four 2.3-kW heaters. Convection heat transfer experiments were conducted with air, nitrogen, and helium for inlet Reynolds number (Re) values ranging from 500 to 70000. Extensive three-dimensional numerical modeling was also performed using a commercial finite element package, COMSOL Multiphysics. The numerical results agreed with the convection heat transfer data, with maximum error percentages under 15%. Based on this agreement, important information was extracted from the numerical model regarding the axial and radial velocity and temperature profiles as well as the axial variations in gas properties. This work examines deteriorated turbulent heat transfer and flow laminarization for a wide range of Re, including laminar, transition, and turbulent flows.