Swirling entrained-bed coal gasifier is able to not only produce fuel gas from coal but also melt ash to slag by high temperature. The molten slag is captured on the wall by swirling flow gas, but sometime a part of the slag is entrained by tip flow gas. At upper zone of the gasifier, gas temperature is lower than ash melting point. Therefore the entrained molten slag to the tipper zone shall solidify and grow. And it disturbs operation. It is necessary to design to lower the height of molten slag entrainment. The slag droplet is transported by the up-flow and the swirling flow. The swirling flow velocity was expressed as a composite of free and forced vortexes. The up-flow velocity was expressed as a function of gasifier exit velocity, These estimates agreed approximately with cold flow model experimental results. Vertical velocity of the slag droplet was calculated from the balance between the up-flow gas velocity and gravity. Tangential velocity was calculated from centrifugal acceleration. The estimated height of slag entrainment agreed approximately with the cold flow model experimental results and EAGLE. pilot-plant results.