In an entrained coal gasifier, coal ash turns into molten slag which deposits on the walls to form solid and liquid slag layers. It is important to understand the slag behavior to prevent the blockage of slag tap and maintain the desired level of heat absorption on the wall. In this study, we proposed a comprehensive numerical model to predict the flow and heat transfer characteristics of the slag layers. The model can be used as the wall condition for detailed computational fluid dynamic or process simulations of the gasifier. The numerical model was evaluated in comparison with existing analytical models for a commercial coal gasifier under three exemplary operating conditions. The new model adequately predicted the thickness, velocity, and temperature of the slag layers, as well as the heat flux to the wall. Under particular operating conditions that had discrepancies between the model predictions, the reasons were discussed in relation to the respective model formulation and assumptions/simplifications introduced. It was shown that the temperature profile within the liquid slag was close to a linear relationship at sufficiently high gas temperatures, while it became parabolic when the gas temperature fell below T-cv. Applying the analytical models requires caution under the conditions in which the assumptions on the temperature profile are not valid. The numerical model can be applied to various operating conditions without such limitations and be extended to incorporate different slag property submodels. (C) 2015 Elsevier Ltd. All rights reserved.