For conducting gas phase chemical reactions in presence of catalyst a shell and tube reactor best suits if heat transfer play important role in the chemical reaction conversion. In case of exothermic reaction, heat evolves as the reaction proceeds producing products from reactant, if this heat is removed than 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 accumulation of heat and when this reaches the activation energy barrier of backward reaction than the reaction ceases and product do not form. Similarly for endothermic reactions heat to be supplied continuously to make reaction proceed forward, we have supply the activation energy.  This of reaction mechanisms demand for better heat transfer designed reactor which can used with catalyst holding, loading, maintenance free and can easily 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