This article presents the results of numerical calculations of chemical composition of flue gas produced by combustion of natural gas in an industrial heating device, i.e., a pusher furnace. In calculations of combustion chemistry, the CHEMKIN-PRO program was used. The gas-dynamic of flue gas was modeled with the use of the COMSOL Multiphysics program. In order to check the correctness of the plug flow reactor (PFR) model, as applied in calculations, a preliminary laboratory experiment was conducted. Based on measurement data that were obtained during the research on a laboratory stand, the boundary conditions were formulated and numerical simulations were carried out. The temperature and velocity profiles in the combustion chamber were determined with the use of the COMSOL program and were implemented in the CHEMKIN program. The obtained results of the numerical simulations performed in the experimental chamber showed high compliance with the results of measurements. This, in turn, led us to apply the analysis on a real object. The simulations of the chemical composition of flue gas in the each zone of the furnace were carried out based on the measurements data collected from the real object, i.e., a pusher furnace that is currently working in one of the steel plants located in our country.