Systems and methods for treating carbon-containing waste materials include a gasifying system, a drier system for pre-drying the material, and an energy-recovery system for recovering waste heat and/or producer gas from the gasifying system/method for use in pre-drying the material. The energy-recovery system can include a recirculation system for recovering the waste heat and/or a thermal oxidizer or other combustion device for burning the producer gas, along with a heat-transfer-loop for transferring the recovered heat energy to the drier for pre-drying the material. In another aspect of the invention, the gasifying systems and methods use a thermal-screw conveyor with a product chamber and rotary thermal screws, and an oxygen-delivery system configured for delivering oxygen into the product chamber for immediate absorption into the material, with or without the dryer system and/or the energy-recovery system.