Numerical study of the turbulent reacting flow in a high-temperature and high-pressure gasifier has been carried out to analyze the characteristics of gasification reaction of heavy residual oil, which has high sulfur content. In this study, the effect of swirl strength of the secondary nozzle and reactor geometry(L/D), known to be the crucial design parameters of gas-fier, has been extensively studied. Especially, we intended to find the effect of operating and design parameters by observing the reacting flow pattern as the key parameters have changed. It is shown that there is progressive increase in the length of central toroidal recirculation zone which plays an important role in flame stabilization and the increase in the reactivity of heavy residual oil as the swirl number increases. In addition, required time for the complete reaction increases and the swirl effect is relatively weakened as the L/D ratio decreases. Most of all, it is found that the optimal combination of L/D ratio and swirl strength is essential to maintain the flame stabilization and efficient mixing ratio of fuel and oxidizer, and to minimize the damage of wall refractory.