Method of online control of a slag-forming process of gasification of a carbonaceous solid fuel, especially coal, in a gasification reactor with supply of a gasifying agent and a moderator, comprising the steps of a) defining the volume of a product gas to be produced by the gasification process, especially of a synthesis gas; b) conducting an online solid fuel analysis on a solid fuel sample taken prior to entry of the solid fuel into the gasification reactor by means of an online solid fuel analyser to determine the ash content and the ash composition of the solid fuel prior to entry into the gasification reactor; c) reading out operating data of the gasification reactor by means of a process control system; d) processing the data from the online solid fuel analysis and the operating data of the gasification reactor in a process model to ascertain an operating point of the gasification reactor while simultaneously achieving the product gas volume defined, wherein the process model i. determines the operating temperature of the gasification reactor based on the ash composition ascertained by the online solid fuel analysis in order to enable essentially complete outflow of slag out of the gasification reactor; ii. determines the required mass flow rate of solid fuel based on the ash content ascertained by the online solid fuel analysis; and iii. determines the required mass flow rate of gasifying agent and moderator based on operating data of the gasification reactor; e) adjusting the operating point of the gasification reactor by means of the process control system for control of the gasification process. a) defining the volume of a product gas to be produced by the gasification process, especially of a synthesis gas; b) conducting an online solid fuel analysis on a solid fuel sample taken prior to entry of the solid fuel into the gasification reactor by means of an online solid fuel analyser to determine the ash content and the ash composition of the solid fuel prior to entry into the gasification reactor; c) reading out operating data of the gasification reactor by means of a process control system; d) processing the data from the online solid fuel analysis and the operating data of the gasification reactor in a process model to ascertain an operating point of the gasification reactor while simultaneously achieving the product gas volume defined, wherein the process model i. determines the operating temperature of the gasification reactor based on the ash composition ascertained by the online solid fuel analysis in order to enable essentially complete outflow of slag out of the gasification reactor; ii. determines the required mass flow rate of solid fuel based on the ash content ascertained by the online solid fuel analysis; and iii. determines the required mass flow rate of gasifying agent and moderator based on operating data of the gasification reactor; i. determines the operating temperature of the gasification reactor based on the ash composition ascertained by the online solid fuel analysis in order to enable essentially complete outflow of slag out of the gasification reactor; ii. determines the required mass flow rate of solid fuel based on the ash content ascertained by the online solid fuel analysis; and iii. determines the required mass flow rate of gasifying agent and moderator based on operating data of the gasification reactor; e) adjusting the operating point of the gasification reactor by means of the process control system for control of the gasification process.