The disclosed embodiments relate to systems for deaerating a stream of slag sump water produced by a gasifier. For example, in one embodiment, a system includes a flash vessel having a first inlet configured to introduce a first fluid into the flash vessel, wherein the flash vessel is configured to flash the first fluid to produce a first flash gas, a second inlet configured to introduce a stream from slag sump into the flash vessel, wherein the stream from slag sump comprises a mixture of a gasification fine slag, dissolved oxygen (O2), and water. A gas-liquid contactor in the flash vessel is configured to contact the stream from slag sump with the first flash gas to enable the first flash gas to deaerate the stream from slag sump. A first outlet of the vessel is configured to output an overhead discharge comprising the first flash gas and oxygen from the stream from slag sump.