A gasification reactor for processing organic waste in batches comprises a primary gasification reactor (62) that includes a primary gasification chamber (18) and a surrounding combustion chamber (19), a secondary gasification chamber (21), a synthesis gas decontamination unit (42) and a combustible gas selector (41). The waste is loaded into the primary gasification chamber through a latched opening and heated from the combustion of a fuel in the combustion chamber (19) to convert the waste to a synthesis gas. The gasification chamber (18) has an intake (20) for introducing pre-heated process air (1) therein. The combustion chamber operates either with a conventional fuel (9) or with the produced synthesis gas (6). The secondary gasification chamber (21) thermally treats the synthesis gas (2) to eliminate tars. The decontamination unit (42) scrubs the synthesis gas of contaminants including particulates and acid gases. The clean synthesis gas (6) is directed to the combustible gas selector (41) which selectively feeds either the combustible fluid (9) or the synthesis gas (6) to the burner (40).