Underground Coal Gasification (UCG) has immense potential for utilization of deep unmineable coals. Simulation of this complex process offers ease in decision-making with significant cost reduction required for commercialization of the technology. The paper demonstrates the prediction of syngas composition and its calorific value (CV) for certain coal by varying inlet O2 concentration \(\left( {C_{{O_{2} }} } \right)\) and reactor temperature (RT) during UCG applying numerical simulation technique. The gasifier is considered as a semi-batch reactor with adiabatic conditions with an initial system volume of 1 m3. The homogeneous and heterogeneous phase reactions were initialized in zero-dimension, considering pyrolysis was completed before the initiation of the process. The RT was fixed at 1200°K and three scenarios were simulated for low, equivalent and high \(C_{{O_{2} }}\) with respect to steam concentrations \(\left( {C_{{H_{2} O}} } \right)\) . In each scenario, the ratio of inlet CO (CCO) to H2 concentration \(\left( {C_{{H_{2} }} } \right)\) was varied <1, 1 and >1 to study evolution of syngas. In the best case scenario at 1200°K for the equivalent \(C_{{O_{2} }}\) and \(C_{{O_{2} }} \,{\text{and}}\,C_{{H_{2} O}}\) with \({{C_{CO} } \mathord{\left/ {\vphantom {{C_{CO} } {C_{{H_{2} }} < 1}}} \right. \kern-0pt} {C_{{H_{2} }} < 1}}\) and \({{C_{CO} } \mathord{\left/ {\vphantom {{C_{CO} } {C_{{H_{2} }} > 1}}} \right. \kern-0pt} {C_{{H_{2} }} > 1}}\) , the syngas compositions obtained were comparable, i.e., CO (31.74 mol%), CO2 (24.87 mol%), CH4 (30.83 mol%), and H2 (12.53 mol%) with a CV of 16.10 MJ/Nm3. However, the UCG process at 1300°K yielded higher CV (16.77 MJ/Nm3) with increased CO and CH4 concentration due to CO2 gasification reaction.