NOVELTY - The reactor tube comprises a fixed bed of Fischer-Tropsch catalyst particles. The catalyst particles in 5-33% of the fixed bed volume at an upstream end of bed have an average outer surface to volume ratio (S/V) of 3.3-4.5 mm-1. A difference between the average ratio of the particles at the upstream end and the average outer surface to volume ratio of the particles in the remaining fixed bed volume is 0.5 mm-1. The catalyst particles in 7-25% or 7-18% of the fixed bed volume at the upstream end have an average outer surface to volume ratio (S/V) of 3.3-4 mm-1.    USE - The reactor tube is useful for performing Fischer Tropsch reaction (claimed) for conversion of hydrocarbonaceous feedstocks into liquid and/or solid hydrocarbons, where the feedstock includes natural gas, associated gas, coal-bed methane, residual oil fractions, biomass and/or coal.    ADVANTAGE - The reactor tube performs the Fischer Tropsch reaction under the optimum operation condition in a safe manner thus enhancing selectivity of the hydrocarbon product, and exhibits long life span and thermal tolerance.    DETAILED DESCRIPTION - The reactor tube comprises a fixed bed of Fischer-Tropsch catalyst particles. The catalyst particles in 5-33% of the fixed bed volume at an upstream end of bed have an average outer surface to volume ratio (S/V) of 3.3-4.5 mm-1. A difference between the average ratio of the particles at the upstream end and the average outer surface to volume ratio of the particles in the remaining fixed bed volume is 0.5 mm-1. The catalyst particles in 7-25% or 7-18% of the fixed bed volume at the upstream end have an average outer surface to volume ratio (S/V) of 3.3-4 mm-1. The catalyst particles in the remaining fixed bed volume have an average outer surface to volume ratio (S/V) of 4.8-7.5 mm-1. A weight of catalytically active metal per volume unit in 5-33% of the fixed bed volume at the upstream end of the bed and a weight of catalytically active metal per volume unit in 7-25% or 7-18% of the fixed bed volume at the upstream end are 64-68% lower than a weight of catalytically active metal per volume unit in the remaining fixed bed volume. The particles in the remaining fixed bed volume have an effective diameter of 1.4 mm. A surface area of the catalytically active metal in the upstream end of the fixed bed is lower than the downstream end. A full-bed apparent catalytic activity per volume unit in 25-50% of the fixed bed volume at the downstream end is 1.5-3 times higher than a full-bed apparent catalytic activity per volume unit in the remaining fixed bed volume. A weight of catalytically active metal per volume unit in 25-50% of the fixed bed volume at the downstream end is 1.5-3 times higher than a weight of catalytically active metal per volume unit in the remaining fixed bed volume. The catalyst particles at the upstream end have an average outer surface to volume ratio (S/V) of 3.3-4 mm-1, and are TL shaped catalyst particles. The catalyst particles in the remaining fixed bed volume have an average outer surface to volume ratio (S/V) of 4.8-7.5 mm-1, and are TA shaped catalyst particles. An INDEPENDENT CLAIM is included for a Fischer Tropsch reaction.