A three-dimensional, four-phase in situ coal conversion simulator is described which rigorously models fluid flow, heat transfer, vaporization/condensation, combustion gasification, pyrolysis, drying, and coal property changes. An Initial version of the simulator is detailed which has five fluid components - water, oxygen, tar, nitrogen, and a product gas. The paper presents the physical/chemical mechanisms which are modeled, the comprehensive mathematical solution, and two sample applications of the simulator to the Hanna Field tests. The applications demonstrate the current capability and future potential of the simulator to interpret field results and predict the sensitivity of field operating procedures. Additional features currently being implemented in the model are described which will enable the simulator to more accurately model the entire process.