How to fix obvious connection line between meshes? I'm experiencing very strange lines on my model. How can I fix this?

First, I take the head object from another model when I combine it appears strange lines do not blend with the face model.
Here's an example.
Model:

Head model:

I tried to merge with (Alt+M&gt; at last) but I can't separate them again.
Here is the picture where I tried to merge:

I want a separate head model like this:
It look like you use 2.79 on one screenshot.
If you want to output it then somewhere from Blender - bake normal maps from models where all parts attached and vertices merged to each separate part. This normal map will be baked with taking in to account normal differences so it will compensate it and make detached parts look similar to full head.
If use just in Blender - use Normal Edit modifier on detached parts where pick full head model with merged vertices as reference object. Like on GIF - 2 separate sphere parts, &quot; normal Object &quot; is full sphere.

I mean use modifier for &quot; non destructive &quot; things. You can also just apply this modifier and not bake any normal maps.