Underground coal gasification (UCG) is a coal conversion process that enables the utilization of coal reserves in-situ, and it is an alternative technique to conventional mining methods. Previous experimental studies showed that UCG is a suitable method for the usage of low-rank coal or lignite reserves, which have the major part in the Turkish coal reserves. In this study, a thermochemical equilibrium model of UCG process is developed to predict syngas composition and to compare UCG-performance of the selected lignite reserves in Turkey. The lignite sites are chosen according to the described UCG site selection criteria. The equilibrium model consists of gasification reactions and water-gas shift reaction and it considers the effect of the drying process. The model is validated using the results of the lab-scale experimental UCG study. The predictions are made for both oxygen and steam-gasification processes. Results show that the lignite reserves which have high moisture content but low carbon content are suitable for hydrogen-rich syngas production and hydrogen production capabilities of all reserves can be enhanced significantly by the additional steam supply as the gasification agent.