To determine the radiant and convective part of heat transfer in fire-tube boilers, the effective temperature in each boiler section must be known. In the case of convective heat exchange, the mean logarithmic temperature difference is required. Previously, for radiation from hot flame and flue gas to a cooled enclosure of simple geometry, such as the case of the cylindrical furnace in fire-tube boilers, an approximate approach has been used to calculate the total radiant heat flow from hot flame and gases to the boiler walls by defining an effective flame temperature. That approach is not based on the laws of physics and was introduced solely because it was simpler than other methods. However, it tends to be inaccurate as the combination of flame and gas radiation can not be accurately assessed. This article introduces a simple method for the calculation of the temperature to be used in assessing the total amount of flame and flue-gas radiation in boilers fired with fossil fuels. The method is suitable for engineers and gives better agreement between measured and calculated values than the previous method.