The results from studying prospective energy technology complexes for producing liquefied natural gas (LNG) and using it at electric power facilities as backup fuel are outlined. The article also gives a description and the main parameters of the original process-flow diagram of the LNG complex for combined heat and power plants of PAO Mosenergo, which operates solely on the plant’s secondary energy resources: the pipeline gas excess pressure and the heat of the closed-circuit cooling system. The complex includes means for pipeline gas liquefaction, long-term storage of the standard backup fuel stock, and LNG gasification using the heat removed from the steam turbine condensers when the power plant is shifted for using backup fuel. Based on an analysis of the experience gained around the world from developing similar complexes and the design parameters of the natural gas liquefaction process-flow diagram proposed by the authors, a procedure for calculating the economic efficiency indicators of the projects of LNG energy technology complexes is developed. Based on the actual data on the consumption of main and backup fuel at PAO Mosenergo, calculations of the technical and economic parameters of LNG complexes were carried out, the results of which have demonstrated the economic efficiency of fuel oil replacement by liquefied natural gas for the options of its production at power plants solely as a commercial product or construction of energy technology complexes instead of planned fuel oil facilities. It is shown that, in view of a low net cost of self-produced LNG and taking into account the rapidly developing market of gas engine fuel, the sales of LNG produced in excess of the standard stock amounts may become a significant item of additional income for a power company. In addition, the construction of additional low-tonnage production facilities equipped with large-capacity LNG storage systems will make it possible to enhance the fuel supply reliability of prospective fueling networks.