The paper presents a methodology of multi-level modeling of thermal-hydraulic conditions of large heat supply systems on the basis of the scientific area of research – the theory of hydraulic circuits – that has been developed at MESI SB RAS. The essentials of the applied methods and also the mechanism of their integration with the up-to-date information technologies are described. In combination they make it possible to calculate operating conditions of heat supply systems (HSS) of arbitrary size and structure. The applied approach is based on the multi-level arrangement of data and single-and multi-level calculations. The latter, in turn, are based on application of the methods of equvalenting and decomposition of both the calculation schemes and problems. The methods for multi-level adjustment thermal-hydraulic calculations take into consideration all requirements for feasibility of conditions and are realized as the informationcomputing system (ICS) “ANGARA-TS”. The problems to be solved using the ICS and the technology for development of operating conditions are presented. The ICS is applied to sets of real HSS of big towns to develop conditions and adjustment measures. The composition of the solved tasks implemented in the ICS, the technology of development of operating conditions and an example of a multilevel modeling of the thermal-hydraulic conditions of the HSS in Petropavlovsk-Kamchatsky are presented.