For calls that depend on previous result you should use concatMap
firstPOSTCallToAPI('url', data).pipe(
concatMap(result1 => secondPOSTCallToAPI('url', result1))
concatMap( result2 => thirdPOSTCallToAPI('url', result2))
concatMap(result3 => fourthPOSTCallToAPI('url', result3))
....
).subscribe(
success => { /* display success msg */ },
errorData => { /* display error msg */ }
);
if your async method does not depend on return value of the precedent async call you can use
concat(method(),method2(),method3()).subscribe(console.log)