The ash deposition behavior of gasifier is compared with the findings in lab-scale DTF experiments and a numerical model will be developed to predict the high-temperature ash behavior in the surface of heat exchanger. Within various physical and chemical properties of coal, mineral component is generally considered as dominant parameter to determine ash deposition. In the development of numerical model, deposition rates of deposit-initiating mineral, such as Na or K compounds are calculated with various conditions. Also, the effect of major component of coal ash on the deposition is coupled with the deposition-initiating rate. Finally, developed numerical model of ash deposition will be refined with the ash deposition behavior of DTF experiment as well as actual pilot-scale experiment.