For conducting gas-phase chemical reactions in presence of a catalyst a shell and tube reactor best suits if heat transfer plays important role in the chemical reaction conversion. In case of an exothermic reaction, heat evolves as the reaction proceeds producing products from reactant, if this heat is removed then the forward reaction continuous because the temperature of the reaction mixture is maintained constant at operating design temperature. If the heat is not removed and so it leads to increase in temperature of the reaction mixture due to the accumulation of heat and when this reaches the activation energy barrier of a backward reaction than the reaction ceases and the product does not form. Similarly for endothermic reactions heat to be supplied continuously to make the reaction proceed forward, we have supply the activation energy.  This of reaction mechanisms demands better heat transfer designed a reactor which can be used with catalyst holding, loading, maintenance free and can easily be operated and controlled. A design in the form of shell and tube heat exchanger where tube acts as plug reactor packed with catalyst and shell the heating or cooling media are used for heat exchange purpose.
The main drawback of shell and tube reactors is that they are not so good for liquid reactions,  but efficient for gas phase catalytic reactions.
Model diagram of shell and tube reactor pid system

shell and tube reactor

Trickle bed reactor