In a certain code language, GERMINATION is written as IMGRENNOAIT. How is ESTABLISHED written in that code?

The middle letter remained unaltered in the code. Let us label the five letters before the middle letter as well as those after it, from 1 to 5. Then, the code contains the letters of each group in the order 5, 4, 1, 3, 2.
Thus, we have :
ESTABLISHED => BAETSLDEIHS
12345/12345 => 54132/54132