In Ukraine, a significant number of domestic and industrial boilers are in operation. In this paper, we present the results of a numerical study of the processes of burning gaseous fuels in a furnace of a steam water-tube boiler DE-10/14. The burner device GMG-7 with a capacity of 728 m3/h of natural gas provides a swirling short and wide torch. The fuel-air mixture is formed by premixing 15% of the air, with a primary twist factor n = 2.4, and a secondary burner twist ratio n = 1.6, and an excess air factor αa = 1.10. The option of installing blades in the primary air duct with an angle φ1 = 45° was considered, and in the secondary air channel with an angle φ2=60°. The burner embrasure is conical with an opening angle of 60°. Twist of primary and secondary air – one way. As a result of the research, the temperature and velocity distributions of gases in the combustion chamber, the density of heat flows on the screen tubular surfaces, and the concentrations of the combustion components were determined. The mathematical model of radiation-convective heat transfer in the gas path of the boiler is formed on the basis of the Reynolds-averaged Navier-Stokes equations with allowance for gravity and neglect of compressibility. The model is the equation of continuity, momentum transfer, energy and chemical components of the gas mixture recorded in a stationary form. The equations are closed by the Newton law for the pressure tensor, the Fourier law for the heat flux, the Fick law for the mass flow, the Clapeyron-Mendeleyev law for the thermodynamic state of the gas mixture, the equations of the Launder-Spaulding turbulence model and the Magnusen-Hertager turbulent combustion model. The simulation is performed by the control volume method. At a slope angle of the blades of the register φ2=60° in the secondary air flow path and φ1 = 45° in the primary air duct, a wide opening of the V-shaped flame near the cutoff of the burner at a distance of 0.5 to 0.6 m is observed. Combustion of the gas – in narrow jets of the open torch at a distance of 1.0 – 1.5 m. The temperature of the gases in the streams of the torch is about 1500 – 1700°C. In this case, narrow jets move near the screen surfaces of the heat exchange tubes. It is established that the structure of the flame at the angle of installation of the blades φ2 = 45° is symmetrical and stable, and the dimensions of the torch correspond more to the geometry of the combustion volume. At the same time, no flare is observed on the screen side surfaces and the bottom of the furnace, tightening the torch in a convective beam. However, the temperature of the gases and the density of the heat flow are not high enough – the average values are about q=100 kW/m2 (at qcp = 76.8 kW/m2).