The effects of furnace design and operating parameters on the thermal performance of a natural gas-fired, high temperature, industrial furnaces have been studied by numerically simulating the flow, combustion and radiative as well as convective heat transfer. The k-e model is used to predict turbulent flow, and Bray's model for premixed combustion is employed. Radiation heat transfer is calculated by accounting for the effects of turbulence/radiation interaction and spectral characteristics of the combustion products. The solution of the model equations is obtained using the SIMPLER algorithm. The results of calculations show that the thermal efficiency of furnace is improved by the increase in the sink emissivity, decrease of sink temperature, oblique firing, and improvement in thermal insulation of refractory walls.