The two-dimensional mathematical model of a temperature field of a crystallizer in which quasistationary processes of internal heat exchange are described by the nonlinear elliptic equations in partial derivatives is constructed. Boundary conditions of convective and radiant heat exchange corresponding to various parts of surface are considered. Stephan conditions for a case of two spatial variables is formulated on interface. A finite-difference approximation that takes account of equations’ nonlinearity is selected for computational solution of the problem. The solution algorithm for the received systems of the equations is developed.