A method for reliquefying boil-off gas generated in a cargo tank of a liquefied natural gas ship generally includes the steps of compressing boil-off gas generated in the cargo tank with a compressor, sub-cooling the compressed boil-off gas in a heat exchanger, feeding the liquefied gas to a gas phase separator, venting flash gas generated in the gas phase separator through a first pipe, feeding the vented flash gas from the gas phase separator to a gas combustion unit of the ship via the first pipe, oxidizing the vented flash gas in the gas combustion unit, diverting a portion of the compressed boil-off gas from the compressor through a second pipe, feeding the diverted portion of compressed boil-off gas from the compressor to an upper region of the gas-phase separator and returning liquefied gas from the gas-phase separator to the cargo tank.