Abstract: An apparatus and method useful for partially condensing vapor in the upper section of a fractionation tower to separate and remove a lighter gaseous fraction from a condensed liquid component, such as nitrogen from natural gas. A downflow, knockback condenser is disclosed that utilizes a vapor riser to introduce a flow of vapor into a headspace above a vertical tubular heat exchanger, thereby establishing a downflow of condensed liquid and a lighter gaseous fraction through the heat exchange tubes.