There are various methods of oil shale thermal processing with the purpose to generate primary energy product so-called shale gas. However, methods that do not require the extraction of rock to the surface (in-situ methods) are of special interest. In these methods, shale gas is extracted directly from the stratum as it is heating up, in one way or another. This paper presents a model of in-situ oil shale heating. In comparison to known papers, this model describes processes under "non-equilibrium" conditions. The model takes into account chemical reactions, the flow of the gaseous decomposition products of in pores, concentration expansion phenomenon and interphase heat and mass transfer. Examples of the numerical realization of two individual tasks are provided. The problems on the reaction initiation in shale plate by heat flux from surface and plane shale layer decomposition under electric heating were examined. It was shown that chemical heat release essentially changes the summary heat balance in the system. The work unveiled several ways of reaction development, including explosion and slow decomposition.