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; 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; - 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, wherein said at least one gasifier sealing apparatus prevents backflow of material from the gasifier; 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; 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; 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; - 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, wherein said at least one gasifier sealing apparatus prevents backflow of material from the gasifier; and - transferring the gasification product gas to said at least one apparatus in group (ii) above.