Seitz made another important save minutes later as Darlington Nagbe raided forward and drilled a low effort towards the bottom corner, but the keeper tipped his strike past the post.
Dallas hit back and came within inches of taking the lead as Je-Vaughn Watson lifted a pinpoint cross into the box for David Texeira, who guided his header at goal, only for Donovan Ricketts to produce a fingertip save.
Portland went ahead just before the break when Nagbe surged forward again, and his shot took a deflection which took the ball past the hand of Seitz and into the net.
In the second half, Texeira should have done better with a strike from close range when Fabian Castillo teed him up inside the box, but he drilled his shot well over the bar.
Portland's hopes rested on the Colorado Rapids defeating the Vancouver Whitecaps, or holding them to a draw, but Kendall Waston's 70th-minute winner fired the Whitecaps into the post-season.