Predicting the spread of smoke from a marine fire in enclosed spaces is still one of the challenge to the functional design of safe and comfortable environments for marine systems. A distinguishing feature of the problem is that the temperature difference as well as the density variation is very large while the smoke flow induced by the heat release is much slower than the propagation of acoustic waves. Therefore, this flow should be considered as a low-Mach-number compressible flow. The generally used Boussinesq approximation, in which the density is assumed constant except for the buoyancy term, is limited to very small temperature difference, and is not an appropriate model for the problem. In this paper, a numerical method is developed, in which the control equations are derived on low-Mach-number approximation, and the Favre averaged compressible k-ε model is used to solve the turbulent transportation coefficients. A two-dimensional thermal plume above a line source is simulated by the present method and the method based on Boussinesq approximation. By comparing the distribution of vertical velocity and temperature above the heat source to theoretical and experimental results, it is found that when the strength of the heat source is very small, the two models give almost the same results. On the other hand, for the case of large strength of the heat source, computation by the low-Mach-number model gives better results than by Boussinesq model.