[en] An effective method for numerical calculation of temperature field in a coolant flowing for example, through a heat exchanger intertube space or in a fuel element cluster is described. The number of arithmetic operations involved has the order of a number of calculating point in a grid area. As an example the results of coolant temperature calculation for an axially symmetrical straighttube steam generator with external radial coolant supply in the upper part of a cluster, practically axial flow in the middle by height section of the cluster and volume heat run-off in the calculational area are presented. The temperature field presented is characterized by considerable temperature gradients. In spite of this the solution requires only 10 interations which testifies to high efficiency of the method