The deal will include around 100 miles of natural gas gathering pipeline with approximate capacity of 600 million cubic feet a day and 53,000hp of compression.
Southwestern will use the sale proceeds to repay the $500m term loan facility that matures in December 2016.
SWN chairman and CEO Steve Mueller said: "With this deal, we are well on our way to achieving the $600m to $800m in divestiture proceeds announced as part of that plan."
Apart from the acquisition, Howard is planning to open a new natural gas gathering system for SWN in Tioga County, Pennsylvania.
Once fully operational, the new system will add up to 380 million cubic feet per day of capacity in the area.
SWN specializes in natural gas and oil exploration, development and production, natural gas gathering and marketing.
President and co-founder of Howard Brad Bynum said: "This is a significant transaction for us as these established assets place Howard Energy Partners in the heart of the prolific Marcellus Shale.
"This acquisition is a testament to our long-term growth strategy to broaden our footprint and build a diversified portfolio of large-scale assets and revenue streams."
The transaction is expected to be completed in the second quarter.
