The deposition of scale on the inner surfaces of the water-wall tubes in the high heat flux regions in a steam boiler furnace can cause serious operation problems. In this paper, a numerical technique for determining the heat flux absorbed by the water-wall tubes, water-steam temperature, and thermal resistance on the inner tube surface from a temperature measured at several interior locations of the tube wall is developed. The scale deposition tube is capable of monitoring changes in the flow of heat transfer caused by scale depositions and changes due to varying furnace conditions. It can work for a long time in the destructive high-temperature atmosphere of a coal-fired boiler. The scale deposition monitor is an online plant monitoring system designed to improve the operation of steam boilers and enhance tube life.