The synthetic gas obtained from the gasification of waste material becomes more important not only in waste reduction but also for the generation of clean energy directly applicable to industrial combustors firing LNG fuel without any major modification of the boiler system. Therefore, in this study, a systematic calculation was made for the turbulent reaction inside a conventional LNG combustor to determine the temperature distribution and fluid flow field. By doing this, the syngas obtained from gasification of combustible waste could be evaluated for the potential applicability of syngas as a substitute for LNG fuel in the industrial field. In this calculation, the ratio of the syngas amount to the LNG amount was fixed. That is, based on calorific value, 70% of the fuel was syngas and 30% was LNG. Since the calorific value of the syngas was different from that of LNG with a high energy density, the different volumetric flow rate was expected to give rise to a visible flow field change together with the local velocity. Thus, in this study, the swirl intensity and the inlet nozzle diameter were varied carefully in order to resolve the flow field and turbulence effects on the reaction characteristics of the co-burning flame. First of all, the calculation result of pure LNG combustion was made successfully as a reference and for evaluation of the code implementation. The results obtained from the numerical simulation of the burning of syngas in the LNG boiler could duplicate the combustion feature almost similar to that of 100% LNG fuels by changing the injection method of the syngas without any major change of the boiler system. The results suggested the high potential of syngas as an economic substitute for conventional LNG fuel.