This undergraduate thesis analyzes the integration of the heat storage tank into the combined cogeneration power plant that runs on natural gas and its impact on the operation of the power plant. The combined power plant connects the gas and steam generation process. The cogeneration plant is used for simultaneous generation of electricity and useful heat energy, which significantly increases the degree of utilization of primary energy compared to the separate production of electricity and heat energy. A mathematical model of the power plant was created, consisting of a gas turbine, heat recovery steam generator, steam turbine and a heat storage tank. The power plant is modelled after data for the unit C of the thermal power plant TETO Sisak, which is obliged to meet the heat consumption of the district heating (DH) system of Sisak. Integration of the heat storage tank allows the thermal power plant to have a greater freedom in choosing the optimal mode of operation because it allows the power plant to produce more heat than the currently required thermal load of the DH system and to store that excess heat in a heat storage tank, which it can later utilize to meet the thermal load of the DH. At a time when high electricity prices are present, the steam can be fully utilized for electricity production, while the heat load can be met from the heat tank and thus can increase revenues. Optimization of the thermal power plant with and without the heat storage tank was made in the MATLAB® program and their work was compared. The main goal of the optimization was to satisfy the district heating thermal load of the power plant and to maximize the profit from the sale of electricity. By using the optimization algorithm "fmincon", in each hour of the characteristic day of the month, the optimum load factor is determined with the imposed limits, with the aim of achieving as much profit as possible in the sale of electricity in the two-tariff market.