Use of an olefinic monomer recovery apparatus for separating and recovering unreacted olefinic monomers after production of a polyolefin resin, the apparatus comprising: a vertical-type heat exchange unit, and a knock-out drum unit, wherein the vertical-type heat exchange unit includes: a heat exchange unit body in which a cooling medium for heat exchange is accommodated; and a plurality of heat transfer tubes which extend in a vertical direction and are installed in the heat exchange unit body so as to be submerged in the cooling medium and through which a gas containing an olefinic monomer and a polyolefin wax passes, and wherein the knock-out drum unit includes: a gas inlet directly connected to the heat transfer tubes of the vertical-type heat exchange unit; a knock-out separating means for separating an olefinic monomer-containing gas and a polyolefin wax-containing liquid from the gas; a first discharge tube, installed at an upper portion of the knock-out drum unit, for discharging the olefinic monomer-containing gas; and a second discharge tube, installed at a lower portion of the knock-out drum unit, for discharging the polyolefin wax-containing liquid, wherein ethylene gas is supplied to the plurality of heat transfer tubes of the vertical heat exchange unit separately from the gas containing the olefinic monomer and the polyolefin wax, the ethylene gas defouls the polyolefin wax or resin on the wall surface of the heat transfer tube and it is separated into the defouled wax or resin and the ethylene gas by the knock-out separating means, and then the defouled wax or resin is discharged to the second discharge tube, and the ethylene gas is discharged to the first discharge tube. a vertical-type heat exchange unit, and a knock-out drum unit, wherein the vertical-type heat exchange unit includes: a heat exchange unit body in which a cooling medium for heat exchange is accommodated; and a plurality of heat transfer tubes which extend in a vertical direction and are installed in the heat exchange unit body so as to be submerged in the cooling medium and through which a gas containing an olefinic monomer and a polyolefin wax passes, and wherein the knock-out drum unit includes: a gas inlet directly connected to the heat transfer tubes of the vertical-type heat exchange unit; a knock-out separating means for separating an olefinic monomer-containing gas and a polyolefin wax-containing liquid from the gas; a first discharge tube, installed at an upper portion of the knock-out drum unit, for discharging the olefinic monomer-containing gas; and a second discharge tube, installed at a lower portion of the knock-out drum unit, for discharging the polyolefin wax-containing liquid, wherein ethylene gas is supplied to the plurality of heat transfer tubes of the vertical heat exchange unit separately from the gas containing the olefinic monomer and the polyolefin wax, the ethylene gas defouls the polyolefin wax or resin on the wall surface of the heat transfer tube and it is separated into the defouled wax or resin and the ethylene gas by the knock-out separating means, and then the defouled wax or resin is discharged to the second discharge tube, and the ethylene gas is discharged to the first discharge tube.