Fouling is a major concern in coal-fired power plants caused by fly ash deposit on the heat exchanger tubes that decreases the overall heat transfer coefficient to water-steam mixture. Fouling has been characterized by weakly bound-loose form, which may be removed by various methods, such as soot-blowing, blast, and sand blowing. We have carried out experimental and modeling work on fouling to develop a methodology by which the thermal conductivity of the ash deposit would be determined in a way similar to the fouling process prevailing in real systems. For that we used tubes identical in material, diameter and temperature to those used in many utility boilers. In the experimental work we placed a tube in an axially symmetric 50 kW furnace, and tested fouling from three coals, bituminous and sub-bituminous. We also developed a dynamic model for the prediction of the ash deposition growth and its heat resistance. Comparison of the model prediction and experimental results yielded satisfactory fit. Consequently, thermal resistance of heat exchanger tuber with ash deposit of those coals was determined.