Underground coal gasification (UCG) involves the conversion of coal in the coal seam into a combustible gas thus avoiding the cost of mining and transportation. The product gas may be served as feed stock for petrochemical industry or fuel gas for power generation. It is potentially a clean method of conversion of coal into a high energy fuel gas and may be adopted to replace small coal fired power plants and to maximize the utilization of coal resources. A cavity is formed by the conversion of coal, and it grows three dimensionally in a non linear fashion as gasification proceeds. Mathematical modeling of the UCG reactor is a challenging task since several phenomena, including heterogeneous reactions, complex flow patterns of reactant gases, thermo-mechanical processes related to the structure of the seam, and so on, occur simultaneously in the UCG cavity. The non-ideal flow patterns in the cavity make a strong impact on the extent of homogeneous gas phase reactions, especially the water gas shift reaction, taking place in the cavity volume thereby influencing the product gas composition (H2/CO ratio) significantly. Thus, considerable computational efforts are required in order to obtain the UCG performance for any given set of input parameters. In this work, we have explore the new approach of compartment modeling that reduces the computational burden on process simulation. The compartment model developed in this exercise is based on the computation fluid dynamics (CFD) simulations and hence residence time distribution of UCG cavity. Copyright © (2009) by the International Pittsburgh Coal Conference.