An ammonia synthesis converter for small production units which provides full access for routine maintenance and catalyst replacement while providing adequate catalyst pressure drop to ensure kinetic performance and reduce heat leak from the catalyst beds. A shell has a removable top head and an annular basket is removably mounted in the shell. First and second catalyst beds are disposed in side-by-side catalyst pods in the annular zone of the basket extending substantially a height of the basket for axial down-flow in series. A quench gas is introduced into effluent from the first catalyst bed and the resulting mixture flows through the annular zone into a top of the second catalyst bed. A feed-effluent interchanger in the inner basket zone is adapted to receive effluent from the second catalyst bed and indirectly heat a feed to the first catalyst bed. Also, methods of operating and servicing the converter.