http://www.citygas.com.sg/forhome/useful-info-2/benefits-of-using-town-gas/

Water vapour represents the bulk of the by-products produced during the combustion of town gas. This is because the main component of town gas is hydrogen and during combustion, hydrogen will react with oxygen in the air to produce heat and water vapour. As a result, town gas emits less carbon dioxide during burning as compared to natural gas and LPG.