A method for the production of at least one product selected from the group consisting of synthesis gas, Fischer-Tropsch synthesis products, power, and chemicals, the method being performed in a system comprising: (i) a dual fluidized bed gasification apparatus comprising: a gasifier and a combustor; at least one particulate separator selected from the group consisting of gasifier particulate separators and combustor particulate separators; and at least one gasifier sealing apparatus selected from: (a) gasifier seal pots comprising a distributor selected from ring distributors, pipe distributors and Christmas tree distributors; and (b) J-valves; and (ii) at least one apparatus selected from the group consisting of power production apparatus configured to produce power from at least a portion of the gasification product gas, partial oxidation reactors configured for oxidation of at least a portion of the gasification product gas, tar removal apparatus configured to reduce the amount of tar in the gasification product gas, Fischer-Tropsch synthesis apparatus configured to produce Fischer-Tropsch synthesis products from at least a portion of the gasification product gas, chemical production apparatus configured for the production of at least one non-Fischer-Tropsch product from at least a portion of the gasification product gas, and dual fluidized bed gasification units configured to alter the composition of at least a portion of the gasification product gas, wherein the method comprises the steps of: - heating a particulate heat transfer material in the combustor to produce a combustor flue gas, wherein the combustor is operated in any one of the following three modes: entrained flow mode, transport bed mode or choke flow mode; - optionally separating heated particulate heat transfer material from the combustor flue gas in at least one combustor particulate separator; - transferring the heated particulate heat transfer material and a carbonaceous feedstock to the gasifier, whereby the heated particulate heat transfer material provides heat for endothermic gasification of the carbonaceous feedstock, to produce a gasification product gas comprising hydrogen and carbon monoxide, and entrained particulate material, wherein said at least one gasifier sealing apparatus prevents backflow of material from the gasifier into said at least one combustor particulate separator; - optionally separating gasification product gas from the entrained product in at least one gasifier particulate separator; and - transferring the gasification product gas to said at least one apparatus in group (ii) above. (i) a dual fluidized bed gasification apparatus comprising: a gasifier and a combustor; at least one particulate separator selected from the group consisting of gasifier particulate separators and combustor particulate separators; and at least one gasifier sealing apparatus selected from: (a) gasifier seal pots comprising a distributor selected from ring distributors, pipe distributors and Christmas tree distributors; and (b) J-valves; and a gasifier and a combustor; at least one particulate separator selected from the group consisting of gasifier particulate separators and combustor particulate separators; and at least one gasifier sealing apparatus selected from: (a) gasifier seal pots comprising a distributor selected from ring distributors, pipe distributors and Christmas tree distributors; and (b) J-valves; and (a) gasifier seal pots comprising a distributor selected from ring distributors, pipe distributors and Christmas tree distributors; and (b) J-valves; and (ii) at least one apparatus selected from the group consisting of power production apparatus configured to produce power from at least a portion of the gasification product gas, partial oxidation reactors configured for oxidation of at least a portion of the gasification product gas, tar removal apparatus configured to reduce the amount of tar in the gasification product gas, Fischer-Tropsch synthesis apparatus configured to produce Fischer-Tropsch synthesis products from at least a portion of the gasification product gas, chemical production apparatus configured for the production of at least one non-Fischer-Tropsch product from at least a portion of the gasification product gas, and dual fluidized bed gasification units configured to alter the composition of at least a portion of the gasification product gas, - heating a particulate heat transfer material in the combustor to produce a combustor flue gas, wherein the combustor is operated in any one of the following three modes: entrained flow mode, transport bed mode or choke flow mode; - optionally separating heated particulate heat transfer material from the combustor flue gas in at least one combustor particulate separator; - transferring the heated particulate heat transfer material and a carbonaceous feedstock to the gasifier, whereby the heated particulate heat transfer material provides heat for endothermic gasification of the carbonaceous feedstock, to produce a gasification product gas comprising hydrogen and carbon monoxide, and entrained particulate material, wherein said at least one gasifier sealing apparatus prevents backflow of material from the gasifier into said at least one combustor particulate separator; - optionally separating gasification product gas from the entrained product in at least one gasifier particulate separator; and - transferring the gasification product gas to said at least one apparatus in group (ii) above.