I hope you mean that the steal beam is under this wall or very close to it. Now I know the ceiling was held up with what was there but while you have the whole wall open, I would bring it up to todays standards. A simple header is two 2x10s nailed together three 3" nails every 12 inches. For a rough opening 36" wide you need a 39" header with a criple under each end and a stud on each end to hold it together. For headers 5 ft and longer add 3" and put 2 criples under each end.
When we have 2 or more openings like you do, we use one long header to cover all the openings.
In order to do this you would build your header and set in front of the wall on the floor and build a temp wall on each side to hold up the ceiling and re and re the wall.
If the beam down stairs is not directly below the wall I would extend the header as long as possible, full length would be best.
In the basement you want solid blocking between the beam and your floor sheeting when your cripples don,t land over a joist.
If the beam isn't right there, I would put solid blocking between the floor joists as in 2x10 pieces 14.5 inches long directly under your wall.
What were after is non squeeky floors and drywall that dosn't crack.