Spiral-coiled tube heat exchangers (SCTHE) have higher heat transfer as compared to the conventional heat transfer devices and are extensively used to extract heat from exhaust gases in the chemical processing industries and also from sunlight for domestic applications. However, no attention has been made to predict heat transfer characteristics considering combined convective and radiative heat transfer in spiral-coiled tubes. In the present study, numerical analysis has been performed to predict fluid flow and heat transfer characteristics by combined forced convection and thermal radiation in spiral-coiled tubes. The P-1 radiation and the renormalized group (RNG) k–ε turbulence models have been used to study the effect of thermal radiation and turbulent convection heat transfer in the spiral-coiled tube, respectively, over a wide range of Reynolds numbers (10,000–100,000) and curvature ratios (0.02–0.05). The emissivity and optical thickness have been varied from 0.0 to 1.0 and 0.0 to 8.0, respectively, to investigate the effect of thermal radiation on heat transfer characteristics in spiral-coiled tubes. For the considered Reynolds number range, it is found that the heat transfer is enhanced by approximately 10% when radiation is taken into account. It is found that the heat capacity increased with an increase in optical thickness and wall emissivity. Further, the effect of optical thickness on fully developed flow is observed weak and the average heat transfer coefficient is influenced by the wall emissivity over the entire flow.