The paper assesses the possibility of replacement of a heating plant with a waste-to-energy (WtE) plant as base heat source, together with a natural gas boiler for covering power peaks and as thermal backup. The decision of the Commission of the EU Member States of July 2017 shows a tightening of the emission limits for NOx, SO2 and other substances for thermal sources with a power over 50 MWt to enter into force in 2021. In the Czech Republic, this restriction may affect a number of heating plants, which are currently in operation and they can become economically unsustainable. In addition to the requirements for greening of obsolete coal-fired heat sources, decreasing cost of electricity or the inappropriate dimensioning of due to current or future heat demand in the district heating system (DHS) can be other factors that affect the economy of these sources negatively. This fact, together with the planned ban on landfilling of untreated municipal waste from 2024 in the Czech Republic, represents the potential for the implementation of WtE projects in localities with DHS. Using the techno and economic mathematical models, the methodology of designing the proper processing capacity of a WtE plant and the gas boiler heat output is discussed. Achieving the best economy is prerequisite of the calculations. Additional boundary conditions are considered, such as the coverage of heat demand or the operating parameters of both heat sources. Furthermore, the economically optimal mode of operation of the two sources is presented on the basis of the operational data from real heating plants. The typical profile of demand for heat during the year and fluctuations at different time levels are taken into account. The paper also summarizes the relationship between key parameters such as the heat price and the available waste treatment cost.