This question may have already been answered by How to setup predictive text (autocomplete) on Mac?, which states:
The closest things I've found to what you are looking for are this
text editor experiment that you have to compile yourself
And this Chrome extension for Gmail https://complete.li
Both would require you to type in their respective places and then
paste into your application of choice, but I haven't found anything
that works systemwide.
However, just to complete the answer, there is a IME type method built into TextEdit, that can be activated by hitting the ESCAPE key, so if you type "beg" then the system will bring up a selection of words beginning with "beg", thus:
However, the title of your question implies that you do not want to use that text editor. However, TextEdit is a system wide text entry mechanism so that particular functionality should be available in a number of text entry fields across the system.
Evernote has an autocomplete system, that relies on the the MAcOS autocomplete.
There is another method, using Emacs, which has an autocompletion mechanism. Again, though, it is tied to use within one particular application, although it is an extremely versatile application, if a little tricky to learn to use.
So, in summary, there appears to be no system-wide IME type functionality, apart rom the one built into OS X's TextEdit mechanism. To re-iterate the answer from Super User: you will have to type in a specific IME friendly application and then paste into your application of choice