Waste management currently is a much-discussed topic. Waste should be treated in accordance with EU Directive 2008/98 EC. However, material recycling cannot be applied to all waste generated. It is not only for this reason that the construction of new waste-to-energy plants (WtEP) is being considered in many countries with high landfilling shares. When designing WtE projects, it is essential to assess their economic sustainability with regard to local conditions. Revenues from the sale of heat are one of the most important parameters for the economy of these facilities in the climatic conditions of Central Europe. This often involves the need to supply heat to an existing central district heating system (DHS). The paper presents a complex mathematical optimisation tool for integrated WtE assessment. It is a multi-period model, where the annual operation is analysed in 365 days. It is able to simulate the simultaneous operation of several heat sources (gas or coal boilers coupled with turbines), while respecting their design constraints and operation economy, such as the power range of individual boilers, turbines, variable operating costs and efficiency, etc. The tool is able to refine the estimate of the acceptable heat price and its amount from the WtEP through the change in the variable costs of heat production before and after the integration of the new WtEP. This is a key benefit compared to commonly used techno and economic models of WtEPs, which in most cases work with a simplified assumption that all heat produced can be utilised if its monthly output is lower than total demand in the DHS. As a result, the calculation of the minimum gate-fee or profitability of the project will be improved. The whole problem is the task of linear integer programming and is implemented in GAMS programming environment with the use of MS Excel for a user-friendly interface. The software functionality is presented in the second part of the paper on a real example of WtEP integration into existing DHS.