An analysis is made of the heat transfer in the turbulent flow of a high-temperature participating gas through a circular cross-section tube whose outer surface is covered by nonideal insulation. The gas is nongray, and its radiative properties are evaluated by the weighted sum of gray-gases model; its transport properties and density are assumed to be uniform throughout the duct. The radiative exchanges are evaluated by the tone method. This work is mainly concerned with the effect of the gas inlet temperature, the tube inner diameter, and the turbulent Reynolds number on the total, convective and radiative Nusselt numbers.