A new on-line method of monitoring the total non-hydrocarbons (TNMHC) from the flue or waste gas of stationary sources, which significantly improves upon the current method commonly used in both Taiwan and China, is described. The major problem with the current method is that higher-boiling-point compounds in the waste gas saturate porous materials within the molecular-sieve column used to immobilize TNMHC and elute methane, which sabotages the fixability of TNMHC and leads to false methane and TNMHC readings during continuous monitoring. Our new method effectively circumvents this issue by incorporating a dual-column and back-flush design, rendering it a true gas chromatography (GC) method. Applying this GC method, light hydrocarbons, such as methane, ethane, and propane, can be fully separated by the dual column set. After methane enters the analytical column, a reverse carrier flow immediately back-flushes the pre-column while keeping the analytical column flowing forward by a second flow of the carrier gas for eluting methane. The field application, particularly upon repeated analyses of flue gas laden with complex and elevated levels of volatile organic compounds (VOCs), has shown that self-cleaning by back-flush is necessary when operating at a constant oven temperature.