NOVELTY - A gasifier system (510) is controlled by detecting a thermal conductivity of a producer gas from a gasifier vessel; and controlling an operating efficiency of the gasifier vessel based on the detected thermal conductivity.    USE - Method for controlling a gasifier system.    ADVANTAGE - The method is economical, involves speedy steps, can measure the thermal efficiency of a gasifier system with a simple instrument, and does not require the lower heating value of the producer gas and fuel feedstock and the fuel consumption rate for measuring gasification efficiency. The gas thermal conductivity of the producer gas can be used as a control variable in controlling the combustion parameters of the gasifier process or gasifier system. Further, the real-time signal from the thermal conductivity cell can be used in conjunction with a process controller to modulate air or other oxidizer such that the efficiency of the gasifier is optimized continuously, and can also be used to shutdown the producer gas supply to heaters or generator engines if the quality of the gas is not to specification as a result of a process upset.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) an apparatus for controlling a gasifier system, comprising a gasifier vessel, a thermal conductivity detector (514) for detecting thermal conductivity of a producer gas from a gasifier vessel, and a controller for controlling an operating efficiency of the gasifier vessel based on the detected thermal conductivity; and    (2) a thermal conductivity detector system for controlling a gasifier system, comprising a producer gas path having a first input and a first output, where the first input receives producer gas from a gasifier vessel and the first output provides a producer gas component to a thermal conductivity sensor; a reference gas path having a second input and a second output, where the second input receives a reference gas and the second output provides an output gas component to the thermal conductivity sensor for providing an output signal related to a thermal conductivity of the producer gas component; and a control system for controlling operation of the gasifier system based on the output signal.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of a process control for a gasifier system.    Gasifier system (510)    Stream sample (512)    Thermal conductivity detector (514)    Gasification temperature information (518)    Fan speed controller (522)