Evolution and combustion of fuel gas discharged into the atmosphere is simulated numerically. Finite-duration vertical releases are considered, the ignition occurring on the axis at some elevation above the source. The main stages of the release and burning are discussed, spatial distributions of concentrations, temperature and reaction rate in the fireball are presented at various stages of evolution, from ignition up to total burnout. Influence of the release parameters (fuel mass, injection velocity, orifice size, ignition source height) on the lifetime of the fireball is studied for methane and propane releases. The dependence of the fireball burning time on the Froude number (defined as the square of the ratio of the release velocity and the characteristic buoyant velocity) is obtained. The effect of the source size and ignition source location is shown to be much less significant. The results of calculations are compared with the experimental data on small-scale methane and propane fireballs. Fairly good agreement on the fireball geometry and burning times is obtained. Applicability limits of the results obtained are discussed.