To understand the behavior of the entrained-flow gasification of organic wastes, high calorie value polypropylene (PP) and low calorie value polyethylene terephthalate (PET) were examined in a down flow furnace. The behavior for operational factors of the gasifier, and the effect of auxiliary fuel were found as: High oxygen-carbon molar ratio of the raw materials were necessary in order to maintain the reaction temperature of 1300°C in these small gasifier of the capacity of 10 kg/h and such a high temperature could not be achieved under the low oxygen-carbon molar ratio. When methane gas was introduced as auxiliary fuel under the low oxygen-carbon molar ratio, reaction temperatures over 1300°C were achieved and higher carbon gasification conversion than 80 % was attained. The shift reaction equilibrium was conducted over 1250°C and the equilibrium constant was approximately 0.85.